A considerable number of compositions have been proposed and are currently marketed for the purpose of modifying the configuration of hair shafts to achieve the appearance desired by hair styling. The modification, in the main, alters the hair shaft by disrupting the disulphide cross-linkages in the keratin fibers, thereby permitting normally wavy hair to be made less wavy or allowing normally straight hair to be reshaped into a waved or curled configuration. A number of particular chemical compounds have been employed, but by far the most commonly used compound is thioglycolic acid or the ammonium, sodium, calcium and potassium salts of thioglycolic acid. The compounds appear to reduce the disulfide cross-link to sulfhydryl groups which allows an easier reorientation of the keratin fibers of the hair shaft. Other thiols and mercaptans can accomplish the necessary cleavage of the disulfide cross-link, but in general are less suitable for the purpose.
Although effective in altering the chemical structure of the hair shaft, the aforementioned organic sulfur-containing compounds are characterized by a generally offensive odor. This is particularly true of the thioglycolic acid compounds. While the odor is tolerable over the relatively short period of the hair treatment, it is extremely difficult to remove these sulfur-containing compounds from the hair and scalp following the treatment to a level where the odor is not detectable.
The common method for overcoming the residual odor problem is to apply a masking fragrance to the hair during and/or after the treatment. Such fragrances must, of necessity, be rather strong, and for that reason can also be objectionable to some people. In U.S. Pat. No. 4,738,841, issued Apr. 19, 1988, to V. P. Pigiet, another means is proposed for reducing the objectionable odor of thioglycolic acid compounds during hair treatment. This means comprises decreasing the amount of thioglycolic acid required by incorporating a thioredoxin or similar dithiol peptide into the treating composition which acts synergistically with a thioglycolic acid compound to disrupt the disulfide bond of hair keratin. This technique doubtlessly decreases the odor level during the hair treatment, but has less effect in lowering the post-treatment level of residual thioglycolic acid compound.